Technical Abstract: Winter cover crops are common components of organic vegetable crop rotations in the central coast of California. The impacts of cover crops variety and seeding rate on weed management are poorly understood. In 2003, a systems study began at the USDA-ARS certified organic research land in Salinas, California, to investigate the effects of cover crop variety and seeding rate on weed population dynamics, soil quality, soil water storage, yield and profitability of an organic vegetable production system. The study includes six cover crop treatments of rye, mustard and a rye/legume mix each planted at a standard (1x) and high (3x) seeding rate. This paper reports cover crop growth dynamics and suppression of weed biomass and weed seed production during the first winter cover cropping period. Measurements included cover crop density, percent ground cover, cover crop and weed biomass production, and seed production by the dominant winter weed (chickweed). Cover crop densities in plants/m2 were 191 (legume-rye 1x), 438 (legume-rye 3x), 238 (mustard 3x), 339 (rye 1x) and 894 (rye 3x). Early season ground cover was higher in the 3x seeding rate for all cover crops, and canopy closure occurred earlier in mustard and rye than in the legume-rye treatments. All cover crop varieties had higher biomass in November at the 3x than at the 1x seeding rate, but by the end of season (March) there was no difference in above ground biomass production. The legume-rye 1x treatment was the least suppressive of weed biomass and chickweed seed production. Weeds growing in the legume-rye 1x produced 11 times more weed biomass and 30 times more chickweed seeds than occurred in the other treatments. These preliminary data suggest that using the legume-rye cover crop at the standard seeding rate will increase the weed seed bank and may exacerbate future weed problems.